To begin with, you must initiate your response to a sniper attack long before one actually occurs. Battles are won in the planning and, if you are to prevail in a sniper attack, you must take a proactive approach to the potential of a sniper attack.
This means that you should take the time to learn from the snipers of the past. Books and articles written about the Who's Who of sniping will reveal a wealth of valuable lessons. Today, we have the advantage of studying the cases of Charles Whitman, Jimmy Essex, James Kristian, Julian Knight, Brenda Spencer, and others like them, from the last 30 years.
I won't repeat the Santayana saying, but much can be learned from the past. The attack profiles and tactics of the shooters are often repeated; likewise, the responses and mistakes made by law enforcement are also running on a tape loop. These case studies provide a textbook for all of law enforcement to study sniper behavior and plan a response. If you take nothing else from this article, become a student of history.
Agencies must establish policies dictating responses of all involved resources. Such policies are already in place for major accidents, natural disasters, and large events. A sniper incident, especially a barricade, requires the same level of involvement and coordination of responding resources. Police, fire, EMS, and other public safety personnel may all be called in during a major sniper call. If no policy is in place today, then you will be making it up on the fly tomorrow.
Plan for the worst-case scenarios, and train for them on a departmental level. It may be a major task to coordinate, and it will be costly and time consuming, but training is the only safe place to try out the elements of your proposed plan and make corrections. Training is the time when mistakes can be made without sacrificing lives.